Diadromy as a Conditional Strategy: Patterns and Drivers of Eel Movements in Continental Habitats
James D. McCleave and Eric Edeline
Abstract.— Diadromy, the change of habitat salinity, is found in many fishes and is often facultative (i.e., not all individuals of a population are diadromous). In some Anguilla spp., Sr:Ca ratios along transects from core to periphery of otoliths have revealed that, within a river basin, some eels enter freshwaters as glass eels and remain there, some remain in coastal or estuarine waters, some move between the habitats after some years, and some move between habitats annually or irregularly. These various patterns of diadromy reflect two fundamentally different behaviors: migration of individuals arriving from the sea and settling at different levels of the watershed, and foraging that is initiated after settlement and may lead to habitat relocations. Migration is an internally initiated process while foraging is essentially environmentally initiated. We depict diadromy as a conditional strategy (i.e., a threshold reaction norm responding to internal cues during migration and to environmental cues during foraging). During the migratory phase, internal cues for being a migrant versus a settler are provided by individual energetic and ontogenetic status while, during foraging, environmental cues for dispersal are provided by agonistic interactions and resource availability. During both phases, the switchpoint for alternative movement decisions is underlain by significant genetic variation so that the conditional strategy may evolve to an evolutionary stable state. We provide a speculative endocrine mechanism for proximate mediation of alternative migratory tactics, which is based on an antagonism between thyroid hormones and a growth hormone-cortisol group. Changes in selective pressures during the past decades (decreased densities in saltwater habitats and increased selection against upstream migrants) may have displaced the stable state of the conditional strategy and thus induced an adaptive drop in the proportion of diadromous individuals among populations.